1. Field of the Invention
The invention relates to a control apparatus and method for an internal combustion engine including a variable valve mechanism which can change at least a lift amount of an intake valve as an opening/closing characteristic of the intake valve.
2. Description of the Related Art
In an internal combustion engine, an intake air amount required when the internal combustion engine is started (a required air amount) varies depending on an engine operating state such as a temperature of the engine. Conventionally, such a required air amount is adjusted by controlling an opening amount of a throttle valve. However, in recent years, various methods for adjusting the required air amount have been proposed, such as a method in which the required air amount is adjusted by changing a lift amount of an intake valve or a valve opened period (a working angle) of the intake valve.
Japanese Patent Application Publication No. 2002-276446 A discloses an apparatus for adjusting an intake air amount when an engine is started. This apparatus includes a lift amount/working angle changing mechanism which can increase/decrease a lift amount and a working angle of an intake valve simultaneously and continuously. The apparatus performs control such that the lift amount and the working angle of the intake valve are made small when the engine is started while the engine is cold.
With this apparatus, since the lift amount and the working angle of the intake valve are controlled to be small when the internal combustion engine is started while the engine is cold, air is take in a combustion chamber at a high speed. Accordingly, atomization of injected fuel is promoted, and a good air-fuel mixture is formed.
With such an apparatus, a good air-fuel mixture is formed by setting both the lift amount and the working angle of the intake valve to small values when the engine is started, especially, when the engine is started while the engine is cold. Accordingly, the air-fuel mixture can be ignited promptly and reliably.
With this apparatus, however, the following problem may occur concerning further improvement of startability. With the apparatus, it is possible to reduce a period until the air-fuel mixture is initially ignited, namely, a period until the internal combustion engine is shifted to an initial combustion state. However, in the state where the lift amount and the working angle are controlled to be small, a sufficient amount of intake air may not be obtained in a period until the internal combustion engine is shifted to a perfect combustion state, namely, a period until the internal combustion engine is shifted to a state where the internal combustion engine can operate by itself. Accordingly, the period from when the internal combustion engine is shifted to the initial combustion state until when the internal combustion engine is shifted to the perfect combustion state may be long due to an insufficient output from the engine.